The present invention relates generally to display systems and, more specifically, to a liquid crystal display with a dimmer.
For displaying electronic images and information, two types of displays are predominant. The first is a cathode ray tube (CRT) and the other is a flat panel display. In a CRT display, such as is commonly used in television sets and computer monitors, red, green and blue light is generated and is mixed together to represent desired colors. In such a scheme, white balance can be carefully controlled by varying the intensity of each of the red, green, and blue light sources. Moreover, the overall intensity of the image can be controlled as well. Thus, CRT displays are capable of producing images with accurate color representation and good intensity. However, CRT displays tend to require a lot of space in order to allow for the tube along which the cathode projects the emissions that cause the phosphorescent glow on the front of the CRT display screen.
By contrast, flat panel displays require very little space. However, they generally suffer from low brightness. One example of a flat panel display is a liquid crystal display (LCD). In high performance applications, the so-called AMLCD (active matrix liquid crystal display) is often employed. Another type of LCD display is the so-called TNLCD (twisted nematic liquid crystal display).
In order to improve the visibility (brightness) of these flat panel displays, it has been known in the art to provide a backlight for directing light through the flat panel display to illuminate it from behind. The backlight is normally constructed of a light-combining wedge designed to be placed closely behind the flat panel display. The light-combining wedge typically is provided with light by multiple light sources (typically white light sources) positioned along the edges of the light-combining wedge.
A typical twisted nematic LCD includes an LCD cell made up of liquid crystal molecules formed in distinct planes between two plates of glass, wherein the two plates of glass are positioned between two polarizing filters having a ninety degree difference in polarization. A backlight (typically a white-light source) is positioned behind the first polarizing filter to provide the necessary luminance. Typically, the liquid crystal molecules in the two outermost planes are positioned at a ninety degree angle relative to each other, with each plane in-between having a slightly different orientation from the plane above and below it, thus causing the polarization of light passing therethrough to twist ninety degrees as it passes through the liquid crystal. As a result, the light exiting the liquid crystals is aligned with the second polarizing filter, thus allowing the light to pass through to the observer. When an electric field is applied across the liquid crystal planes, the liquid crystal molecules realign themselves so that they are parallel to the electric field. Consequently, the polarized light passes through the liquid crystals without being twisted, thus contacting the second polarizing filter at a perpendicular polarization. In this state, the second polarizing filter acts as a shutter for individual pixels in the display and blocks the polarized light.
In high performance applications, an active matrix liquid crystal display (AMLCD) is often employed. In a commonly utilized AMLCD, switching transistors, known as thin film transistors (TFTs), activate each individual LCD pixel making it possible to control all pixels together at one time.
In the use of such flat panel displays, it is often desirable to control the dimming of the backlighting for various application settings, such as day/night operations. To achieve such an effect under the known prior art methods, either multiple lamps for separate day/night operation are utilized or the lamps are electronically controlled to vary the light intensity, thereby substantially increasing the cost and complexity of LCD""s.
Accordingly, a new and improved dimmer for LCDs is needed that does not require the use of additional lamps or complex electronics. It is to the provision of such an improvement that the present invention is primarily directed.
Briefly described, in a first preferred form, the present invention both overcomes the above-mentioned disadvantages, and meets the recognized needs for such a device, by providing a light valve dimmer apparatus and method of use thereof, generally comprising a twisted nematic liquid crystal cell utilized in conjunction with an AMLCD. The present invention uses a twisted nematic cell not as a display, but instead as a dimmer device. Thus, the invention combines two LCD devices, one for use as a dimmer and one for use as a display device.
Preferably, the present invention comprises a backlit liquid crystal display apparatus which includes an LCD display panel and a light source positioned behind the LCD display panel. The liquid crystal display apparatus further includes a dimmer device positioned between the light source and the LCD display panel. The dimmer device includes a first polarizing element only. The dimmer device further includes a twisted nematic LCD cell positioned between the first polarizing element and the LCD display panel.
Preferably, the LCD display panel is an active matrix LCD display. Also preferably, the twisted nematic LCD cell comprises a single pixel cell which is sized to shroud the LCD display panel. Most preferably, the twisted nematic LCD includes an active area which is substantially matching in size to an active area of the LCD display panel.
More specifically, in a preferred embodiment, the TN liquid crystal cell is positioned between polarizing material, such as DBEF or DRP material, and the rear of the AMLCD, adjacent to the rear polarizer of the AMLCD. The backlight is placed behind the polarizing material to provide the necessary light source. As such, when the polarized light enters the non-energized TN liquid crystal cell, the polarization of the light is twisted ninety degrees and is thus, perpendicular to and misaligned with the rear polarizer. In this state, light is completely prevented from entering the AMLCD.
As an electric field is applied across the TN liquid crystal cell, the liquid crystal molecules partially or fully align with the electric field depending on the voltage applied. In the fully aligned state, the polarized light is not twisted by the TN liquid cell and is thus aligned with the rear polarizer of the AMLCD, thereby allowing the polarized light to enter the AMLCD. In the partially aligned state, the polarized light entering the TN liquid crystal will be partially twisted, thereby reducing the amount of light that is aligned with the rear polarizer of the AMLCD. As a result, the light is dimmed as it enters the AMLCD.
Accordingly, it is an object of the present invention to provide a liquid crystal display with dimmer apparatus and method that can function to block light partially, to block light fully, or to allow light to enter an LCD unimpeded.
It is another object of the present invention to provide a liquid crystal display with dimmer apparatus and method that can be incorporated into an LCD.
It is yet a further object of the present invention to provide a liquid crystal display with dimmer apparatus and method that does not require the use of additional lamps.
It is yet a further object of the present invention to provide a liquid crystal display with dimmer apparatus and method that does not require the use of complex electronics.
It is yet a further object of the present invention to provide a liquid crystal display with dimmer apparatus and method that is energy efficient and thus inexpensive to operate.
It is yet a further object of the present invention to provide a liquid crystal display with dimmer apparatus and method that is relatively impervious to temperature changes or humidity.
It is yet a further object of the present invention to provide a liquid crystal display with dimmer apparatus and method having high light transmission and low light loss.